"Tailgate" is not actually Italian, believe it or not, but it is actually a slang term for glissando (also known as "glissing" or "smearing"), or the unique ability of the trombone to slide between two notes. This term is used to denote a Dixieland style of smear. The Virgina Tech Multimedia Music Dictionary gives this etymology for the term "tailgate":
And here you thought tailgating only had to do with football gameday activities. Hah!The name tailgate comes from the late 19th century when the trombone performers in Dixieland bands would often perform sitting on or over the "tailgate" of a horse-drawn wagon, facing backwards, while the other members of the band sat in the wagon bed in a parade or in a procession on the way to a funeral. By sitting on or over the "tailgate", the trombone player had ample room to move the trombone slide while performing frequent glissandos of the Dixieland jazz that often require the performer to extend the slide to its maximum length.